Journal of Ceramic Processing Research. Vol. 8, No. 5, pp. 305~311 (2007)

305

J O U R N A L O F

CeramicProcessing Research

Growth, structure and optical properties of amorphous or nano-crystalline ZnO

thin films prepared by prefiring-final annealing

Kyu-Seog Hwanga, Yun-Ji Leea,b and Seung Hwangbob,*

aDepartment of Applied Optics and Institute of Photoelectronic Technology, Nambu University, 864-1 Wolgye-dong, Gwangsan-

gu, Gwangju 506-824, KoreabMajor in Photonic Engineering, Division of Electronic & Photonic Engineering, Honam University, 59-1 Seobong-dong,

Gwangsan-gu, Gwangju 506-714, Korea

The structural, surface morphological, and optical characteristics of amorphous or nanocrystalline ZnO thin films depositedon soda-lime-silica glass substrates by a prefiring-final annealing process at various temperatures were investigated using X-ray diffraction analysis, a field emission-scanning electron microscope, a scanning probe microscope, a ultraviolet-visible-nearinfrared spectrophotometer, and photoluminescence (PL). The films exhibited an amorphous pattern even when finallyannealed at 100 oC-200 oC for 60 minutes, while highly c-axis oriented ZnO films were obtained by prefiring at hightemperature, 400 oC and 500 oC. A relatively high transmittance in the visible spectra range and clear absorption edges of thefilms were observed except for the film annealed at 100 oC. The PL spectra of amorphous ZnO thin films annealed at 100 oC-200 oC with strong near-band-edge (NBE) emission were observed while the defect-related broad green emission was nearlyfully quenched. After high-temperature prefiring and final annealing at above 300 oC, the PL spectra of the films were greatlydeteriorated, the UV peak was diminished.

Key words: ZnO thin films, Amorphous pattern, Transmittance, NBE emission.

Introduction

Zinc oxide based coatings are of much interest inscience and technology due to their interesting potentialapplications [1-3], such as in thermoelectric and gassensor devices, as transparent electrodes, selective sur-faces, piezo-electric devices, etc. The wide range ofapplications is a result of the fact that ZnO is both apiezo-electric and electro-optic (EO) material, and asemiconductor which possesses a wide band gap (3.3eV) [4]. The most unique property of ZnO is its largeexciton binding energy of 60 meV, which is much largerthan those of GaN (24 meV), ZnSe (19 meV) and ZnS(39 meV) [5]. Because of this large binding energy, theexciton is stable at room temperature even in bulkcrystals. Owing to these properties, ZnO is consideredto be a promising material for light-emitting devicesand semiconductor lasers with low thresholds in theultraviolet (UV) region. Generally, the correspondingPL spectra obtained from ZnO thin films shows defect-related deep-level emission [6, 7] (yellow-green emissionaround 510 nm and red emission around 650 nm) aswell as UV NBE emission around 380 nm, whichstrongly depends upon the preparation methods and

growth conditions. Using molecular beam epitaxy (MBE), RF magnetron

sputtering, metal organic chemical vapor deposition(MOCVD) and other methods, high quality ZnO layershave been grown and their structural and optical pro-perties have been extensively studied [8-11]. However,most of the reports on the UV emission of ZnO filmshave concentrated on high-vacuum processes which arevery expensive methods from the viewpoint of thesystem and source materials. To meet the industrialneeds for commercially available ZnO devices, easierand cheaper deposition methods for ZnO films need tobe developed. Chemical solution deposition (CSD) isanother attractive technique for obtaining thin filmsand has the advantages of easy control of the filmcomposition and easy fabrication of a large-area thinfilm at low cost [12-15]. Only a few researchers havereported an accompanied single violet emission of ZnOprepared by CSD [16].

In this study, we first report a simple and efficientmethod to prepare amorphous or crystalline ZnO thinfilms with pure strong UV emission on soda-lime-silicaglass (SLSG) substrates by low-temperature annealing.

Experimental

A homogeneous coating solution was prepared bymixing Zn acetate [(CH3COO)2Zn·2H2O] (Merck, Ger-many) and 2-methoxyethanol (HOCH2CH2OCH3) (Merck,

*Corresponding author: Tel : +82-62-970-5495Fax: +82-62-943-5495E-mail: hbs@honam.ac.kr

306 Kyu-Seog Hwang, Yun-Ji Lee and Seung Hwangbo

Germany). Since Zn acetate has a low solubility in 2-methoxyethanol, 2-aminoethanol (H2HCH2CH2OH)(MEA) (Merck, Germany) was added to obtain a clearsolution (concentration: 0.6 mol Zn acetate/l 2-meth-oxyethanol). The molar ratio of MEA to Zn acetatewas fixed at 1.0. The mixed solution was stirred for 2 hto obtain a homogeneous sol.

Prior to the coating process, SLSG substrates werecleaned in deionized water, immersed in an H2O2 solu-tion, and finally rinsed in acetone.

The starting solution was spin-coated onto the clean-ed substrates at 4000 rpm for 10 s in air. The as-deposited films were prefired at 100, 200, 300, 400,and 500 oC for 10 minutes in air. The coating processwas repeated 13 times to prepare a thick coating ofZnO. Then the final annealing was performed in air at100, 200, 300, 400, and 500 oC for 60 minutes. Figure1 illustrates the processing scheme for preparation ofZnO thin films.

The thicknesses of the finally annealed ZnO thinfilms were approximately 0.5-0.7 μm, as determined byobservations of fracture cross-sections with a fieldemission-scanning electron microscope (FE-SEM, S-4700, Hitachi, Japan). Since the defects in the finallyannealed films were largely affected by the eliminationmodes of the organics in the precursor during the CSD-based process and the structural and optical propertiesare vital for semiconductor devices especially for lightemitting devices, it is necessary to study how theseproperties are affected by prefiring and thermal anneal-ing. Thermogravimetric analysis (TGA, DTG-60, Shimadzu,Japan) of the coating sol was performed. The crystalli-nity of the ZnO thin films was investigated by using ahigh resolution X-ray diffraction technique (HRXRD,X’pert-PRO, Philips, Netherlands). A CuKα (λ=1.54056Å) source was used, and the scanning range was bet-ween 2θ=20o and 70o. The surface morphology of thefilms was evaluated from FE-SEM micrographs. Thegrowth mechanism and the surface roughness of the

thin films were studied using a scanning probe micro-scope (SPM, PSIA, Republic of Korea). All SPMmeasurements were performed in air using the tappingmode. The transmittance in the visible range was mea-sured using a UV-visible-NIR spectrophotometer (CARY500 Scan, Varian, Australia). The transmittance wasautomatically calibrated against that of a bare SLSGsubstrate as a reference sample, and the absorptioncoefficient was obtained from the transmittance curve.Room temperature PL spectra of the samples weremeasured by a micro-PL system (LabRamHR, JobinYvon, France) using the 325 nm line of a He-Cd laseras the excitation source.

Results and Discussion

Figure 2 shows a TGA curve of the coating sol usedin this study. A larger weight loss corresponding topyrolysis of the starting sol began around 130 oC andwas completed just below about 200 oC, as shown inFig. 1. The TGA curve of the starting sol (heating rate:2 K·minute−1) dried at 80 oC for 24 h showed a largeweight loss due to the vaporization and pyrolysis oforganics was recognized in the stage of pyrolysis bet-ween 150-200 oC. Therefore, pyrolysis of the startingsolution is complete below about 200 oC. Thus, toinvestigate the effects of temperatures during prefiringand final annealing, we prepared ZnO films via theabove- mentioned prefiring and annealing conditions in

Fig. 1. Flow chart of the experimental procedure.

Fig. 2. TGA curve of the coating sol used in this work.

Fig. 3. HRXRD spectra of the ZnO thin films on SLSG substratesheat treated at various temperatures.

Growth, structure and optical properties of amorphous or nano-crystalline ZnO thin films 307

our experimental procedure. Figure 3 shows the XRD curves of ZnO thin films

deposited on SLSG substrates. A strong (002) peak isobserved at 2θ=34o for the samples heat treated atabove 300 oC. This indicates ZnO thin films prepared atabove 300 oC by using the CSD process with the zincacetate-2 methoxyethanol-MEA solution show a c-axisorientation, i.e., a growth vertical to the substrate sur-face. The c-axis orientation in the ZnO films preparedusing a physically dry method can be understood by the‘grain-boundary movement’ model proposed by Loderet al. [17]. According to this model, at the very firststage of film growth, certain grains with particularorientations start to grow. By thermodynamical coale-scence of crystallites during film growth, oriented growth(c-axis orientation for ZnO films) is achieved.

From the report of Ohyama et al. [18], for ZnO filmsprepared using a CSD process with zinc acetate-2methoxyethanol-MEA solution, removal of the solventand the organic substances produced by acetate decom-position prior to crystallization may be one of the keyfactors that provides oriented crystal growth. Thevaporization of the solvents, the decomposition of thezinc acetate, and the crystallization of the zinc oxidemay occur almost simultaneously when the heating rateis high. Since the structural relaxation of the gel film,which is induced by the solvent vaporization and acetatedecomposition, can take place only before the crystalli-zation, the simultaneous vaporization, decomposition,and crystallization may give the film less chance to bestructurally relaxed [18].

On the other hand, when the heating rate is low, thegel film is given enough time to structurally relaxbefore crystallization, resulting in denser ceramic films.Moreover, when the prefiring temperatures are too high(> 300 oC), vaporization of the solvents, and thermaldecomposition of the zinc acetate may take place abrupt-ly and simultaneously with the crystallization, disturb-ing the unidirectional crystal growth [18].

However, in this study, it should be noted that c-axisoriented ZnO thin films were obtained by prefiring at ahigh temperature, 400 and 500 oC, and heating at a highrate. This result suggests that structural relaxation ofthe precursor gel before crystallization, by heating thefilm at low rates or prefiring at low temperatures of200-300 oC, is unessential for obtaining oriented ZnOfilms.

The films show an amorphous pattern even whenfinally annealed at 100-200 oC for 60 minutes, whichimplies the most of the processes occurring up to 200oC are mainly related to removal of organic compoundsin precursor, as shown in Fig. 2.

We also observed that with an increase in heattreatment temperature, the full widths at half-maximum(FWHM) decrease from 0.64o at 300 oC to 0.37o at 500oC, while the peak intensity shows an increase withannealing temperature, as shown in Table 1. These results

indicate that the ZnO thin film prepared with zincacetate-2 methoxyethanol-MEA solution and heat treat-ed at high temperature can be expected to have highcrystallinity. The increase in intensity of the (002) reflec-tion at 400 oC and 500 oC is a direct indication of thelarge volume of material oriented with (002) planesperpendicular to the SLSG substrate surface.

On the basis of the XRD data, the lattice c parameterhas been estimated to be 5.2700 Å, 5.2096 Å, and5.1963 Å at 300 oC, 400 oC, and 500 oC, respectively, asshown in Table 2. These values are similar to theASTM value of 5.2066 Å for bulk ZnO. The largervalues of the lattice constant for the films heat treatedat 300 oC and 400 oC compared to the standard powdervalue shows that the unit cell is elongated along the c-axis, and that compressive forces act in the plane of theZnO film. These compressive forces disappear as theheat treatment temperature is increased to 500 oC.

In order to investigate more exactly the structuralproperties of ZnO films, we calculated the stress in thefilm. The calculation of the film stress is based on thebixial strain model [19]. Film strain εz=(c–c0)/c0, wherec0 is the strain-free lattice parameter measured fromZnO powder sample. To drive the stress of the filmparallel to the film’s surface, we used the followingformula [19], which is valid for a hexagonal lattice:

σ =[2c213/2c13–c33(c11+c12)/2c13]ez (1)

where cij are elastic stiffness constant for ZnO. Theelastic constants used were: c11=208.8 GPa, c33=213.8GPa, c12=119.7 GPa, and c13=104.2 GPa [20]. The stressof the film can be estimated using Eq. (1) and is plottedas a function of the heat-treatment temperature in Fig.4. The negative sign for the film heated at 300 oC and400 oC indicates that the lattice constant c is elongatedas compared to the unstressed powder; therefore, the

Table 1. The peak intensity and FHWM of the (002) reflectionfrom ZnO/SLSG

Temperature Peak intensity (cps) FWHM (o)

100 oC − −

200 oC − −

300 oC 295 0.64

400 oC 930 0.50

500 oC 2296 0.37

Table 2. Lattice parameter c and d-length calculated from theXRD patterns of ZnO (002) reflection

Temperature d c

100 oC − −

200 oC − −

300 oC 2.6085 Å 5.2700 Å

400 oC 2.6048 Å 5.2096 Å

500 oC 2.5968 Å 5.1963 Å

Standard ZnO 2.6033 Å 5.2066 Å

308 Kyu-Seog Hwang, Yun-Ji Lee and Seung Hwangbo

film is in a state of elongation. The film becomesnearly stress free at 500 oC.

From the FWHM and the peak position of theZnO(002) peak, the grain size D was derived from thewell-known Scherrer’s relation [21] as below:

D=kλ/Bcosθ (2)

where, λ=1.54056 Å is the wavelength of the CuKαradiation, k=0.9 is the correction factor, B is theFWHM of the ZnO(002) peak, and θ is the diffractionangle. The grain size increases with an increase in theannealing temperature above 300 oC from 12.96 nm at300 oC to 22.19 nm at 500 oC.

Figure 5 shows FE-SEM micrographs of ZnO thinfilms after heat treatment at (a) 100 oC, (b) 200 oC, (c)300 oC, (d) 400 oC, and (e) 500 oC, respectively. Aparticulate structure is indistinct in all films exceptfilms treated at 400 oC and 500 oC. The FE-SEM imageof the ZnO film prepared at 200 oC shows a differentmorphology compared to the other images; some crack-like texture probably due to vaporization of organics isvisible on the surface of the film. There is no evidence

of aggregation of particles and nano-sized particles wereobtained in the films annealed at 400 oC and 500 oC.

Figure 6 shows typical SPM micrographs (1 μm×1μm) of ZnO thin films obtained at the annealingtemperatures of 100 oC, 200 oC, 300 oC, 400 oC, and500 oC. We can observe many regular and uniform grainsin the SPM images of the films annealed at 300 oC,400 oC and 500 oC. In the amorphous films annealed atlow temperatures, below 200 oC, a relatively rough surfacestructure probably due to vaporize residual organicsduring annealing as seen in Fig. 6(a) and (b). As theannealing temperature increases to 500 oC, the surfacemorphology becomes rough by grain growth [Fig. 6(e)].

UV transmission measurements were carried out foroptical characterization of the films. Fig. 7 shows thevisible spectra in the wavelength range from 340 nm to900 nm of ZnO thin films annealed at various temper-atures on SLSG substrates. A relatively high transmi-ttance in the visible spectral range and clear absorptionedges of the films were observed except for the filmannealed at 100 oC. The high transmittances of the filmsare attributed to the small particle size which eliminateslight scattering [22]. The transmittance in the UV spec-tral region decreased abruptly near 3.2-3.3 eV, resultingfrom band-to-band transition. In this transition, UVabsorption occurs due to the excitation of electronsfrom the filled valence band to the conduction band.

The optical absorption coefficient α of the films canbe calculated from the transmittance using the relation-ship:

I=I0e−αt (3)

where I is the intensity of the transmitted light, I0 is theintensity of the incident light, and t is the thickness ofthe ZnO film. As the transmittance is defined as I/I0,we obtain α from Eq. (3). It is well known that the

Fig. 4. Variation of the stress induced in the ZnO films as afunction of the heating temperature.

Fig. 5. FE-SEM images of ZnO thin films on SLSG substrates heat treated at various temperatures.

Growth, structure and optical properties of amorphous or nano-crystalline ZnO thin films 309

absorption coefficient α for allowed direct transitions ata given photon energy hν can be expressed as:

α ≅ (hν−Eg)1/2 (4)

where, h is Plank’s constant, ν is the frequency of theincident photon, and Eg is the optical energy band gapof the film. By plotting α2 versus hν and extrapolatingthe linear position of the curves to α2 is zero gives Eg.The estimated values of the band gap for ZnO filmsannealed at various temperatures by this extrapolationmethod [23] are 3.34 eV, 3.25 eV, 3.28 eV and 3.30 eVat 200 oC, 300 oC, 400 oC, and 500 oC, respectively,close to the instinsic band gap of ZnO (3.2 eV).

The PL spectra at room temperature of amorphous ornano-crystalline ZnO thin films on SLSG substrates

obtained by pyrolysis and annealing at 100 oC, 200 oC,300 oC, 400 oC, and 500 oC are shown in Fig. 8 andTable 3. In the PL spectra, for the films annealed atbelow 200 oC, only a strong NBE emission is seen.This NBE peak has been previously attributed to theemission from a free exciton in the literature [24]. Theintensity of NBE emission peaks increase with an increaseof annealing temperature up to 200 oC and rapidly de-crease again with an increase of annealing temperaturefrom 300 oC to 500 oC. After high-temperature annealingabove 300 oC, the PL spectra of the film were greatlydeteriorated, and the UV peak was diminished. The PLintensity decreases at high-temperature by a thermally-activated non-radiative recombination mechanism. Choiand Ma [25] reported that the non-radiative recombi-

Fig. 6. Three-dimensional SPM images of ZnO thin films on SLSG substrates heat treated at various temperatures.

Fig. 7. UV-visible spectra of ZnO thin films on SLSG substratesheat treated at various temperatures.

Fig. 8. The PL spectra at room temperature for ZnO thin films onSLSG substrates heat treated at various temperatures.

310 Kyu-Seog Hwang, Yun-Ji Lee and Seung Hwangbo

nation centres were generated by oxygen vacancies whichincrease with increasing temperature. For the filmannealed at 200 oC, the largest NBE peak intensity inthe PL spectra was observed. This indicates that anannealing temperature of 200 oC is an optimum condi-tion for the formation of a ZnO thin film with a strongsingle NBE emission. The defect-related broad green(deep-level) emission is not seen in all the samplesexcept for the film annealed at 500 oC. The origin ofthe green luminescence is still in dispute, but it isusually attributed to emission related to grain boundarydefects and other interior defects such as oxygenvacancies (VO) and impurities [26].

Apparently, the degradation of PL is in contrast withthe XRD results, which have shown an improvement ofcrystal quality of the films. The crystallinity of the ZnOfilms annealed at 400 oC and 500 oC was superior tothat of films annealed at low temperature, whereas thePL properties of the ZnO films annealed at 100 oC and200 oC were better than those annealed at higher temper-atures. For the film annealed at a low temperature, 200oC, the FWHM value of the PL spectrum curve was28.7 meV and this value is believed to be smaller thanany previously reported value of ZnO films preparedby CSD. Several important properties based on ZnO,such as electroluminescence (EL) and PL, are stronglyrelated to the defect formation. Many researchers report-ed that green-orange PL emitted from ZnO films isprobably due to different point defects [27] and strong-ly dependent on the predominant defect, which affectsthe deep-level emissions. Vanheusden et al. [28] andChen et al. [5] reported that oxygen vacancies areresponsible for the green emission. However, Zhang etal. [29] and Lin et al. [30] considered the green emi-ssion of ZnO films to be due to zinc vacancies andantisite Ozn, respectively.

The PL spectra of amorphous ZnO thin films withstrong UV emission was observed while the visibleemission was nearly fully quenched. These features canbe explained reasonably by Wang et al. as follows [16]:(i) The higher degree of disorder likely leads to strongeremission than that of crystalline ZnO. (ii) Quantumconfinement effects (QCE) occur when the particleradius is of the order of the exciton Bohr radius (1.8nm, ZnO). Although it is difficult to obtain the exactdiameter of amorphous ZnO, we assume that the sizes

of amorphous ZnO are smaller than 1.8 nm. It is well understood that PL spectra depend on the

stoichiometry and the microstructure of a film. There-fore, these results indicate that the amorphous ZnOfilms obtained at low temperature are very close tostoichiometry and of optically high quality. Our find-ings show that the PL property of the ZnO thin films isimproved because the grain size decreases with low-temperature annealing.

Conclusions

In this study, amorphous or nano-crystalline ZnO thinfilms were grown on inexpensive SLSG substratesusing a CSD process with a zinc acetate - 2 methoxy-ethanol - MEA solution. From XRD analysis, the filmsexhibited an amorphous pattern even when finallyannealed at 100 oC-200 oC for 60 minutes, while highlyc-axis oriented ZnO were obtained by prefiring at ahigh temperature, 400 oC and 500 oC. A relatively hightransmittance in the visible spectral range and clearabsorption edges of the films were observed except forthe film annealed at 100 oC. From the PL measurements,a strong NBE emission was observed for the ZnO filmsannealed at low temperature, below 200 oC, while thedeep-level emission is almost undetectable except forthe film annealed at 500 oC. These results indicate thatit should be possible to cheaply and easily fabricateZnO-based optoelectronic devices at low temperature,below 200 oC, in the future.

References

1. K.S. Hwang, Y.S. Jeon, B.A. Kang, J.H. An, and B.H.Kim, J. Ceram. Proc. Res. 5[4] (2004) 313-315.

2. B.H. Kim, J.H. An, Y.S. Jeon, J.T. Jeong, B.A. Kang, andK.S. Hwang, J. Mater. Sci. 40 (2005) 237-239.

3. J. Bian, X. Li, L. Chen, and Q. Yao, Chem. Phys. Lett. 393(2004) 256-259.

4. J.B. Webb, D.F. Williams, and M. Buchanan, Appl. Phys.Lett. 39 (1981) 640-642.

5. Y.F. Chen, D.M. Bagnall, H.J. Hoh, K.T. Park, K. Hiraga,Z. Zhu, and T. Yao, J. Appl. Phys. 84[7] (1998) 3912-3918.

6. B. Zhao, H. Yang, G. Du, X. Fang, D. Liu, C. Gao, X. Liu,and B. Xie, Semicond. Sci. Technol. 19 (2004) 770-773.

7. I.S. Kim, S.H. Jeong, S.S. Kim, and B.T. Lee, Semicond.Sci. Technol. 19 (2004) L29-L31.

8. H. Tampo, A. Yamada, P. Fons, H. Shibata, K. Matsubara,K. Iwata, S. Niki, K. Nakahara, and H. Takasu, Appl. Phys.Lett. 84[22] (2004) 4412-4414.

9. K.K. Kim, J.H. Song, H.J. Jung, S.J. Park, J.H. Song, andJ.Y. Lee, J. Vac. Sci. Technol. A 18[6] (2000) 2864-2868.

10. W.I. Park, S.J. An, G.C. Yi, and H.M. Jang, J. Mater. Res.16[5] (2001) 1358-1362.

11. L. Wang, Y. Pu, W. Fang, J. Dai, C. Zheng, C. Mo, C.Xiong, and F. Jiang, Thin Solid Films 491 (2005) 323-327.

12. Y. Natsume, and H. Sakata, Thin Solid Films 372 (2000)30-36.

13. R. Ayouchi, D. Leinen, F. Martin, M. Gabas, E. Dalchiele,and J.R. Ramos-Barrado, Thin Solid Films 426 (2003) 68-

Table 3. The peak intensity, the peak energy and FHWM of theNBE emission peak from ZnO/SLSG

Temperature Peak intensity

(cps) Peak energy

(eV) FWHM

100 oC 242020 3.10 50.8

200 oC 376098.6 3.14 28.7

300 oC 78168 3.13 24.9

400 oC − − −

500 oC − − −

Growth, structure and optical properties of amorphous or nano-crystalline ZnO thin films 311

77. 14. P. Nunes, E. Fortunato, and R. Martins, Thin Solid Films

383 (2001) 277-280.15. F. Paraguay D., J. Morales, W. Estrada L., E. Andrade, M.

Miki-Yoshida, Thin Solid Films 366 (2000) 16-27.16. Z. Wang, H. Zhang, Z. Wang, L. Zhang, J. Yuan, S. Yan,

and C. Wang, J. Mater. Res. 18[1] (2003) 151-155.17. J.C. Loder, T. Wielinga, and J. Worst, Thin Solid Films

101 (1983) 61-73.18. M. Ohyama, H. Kozuka, and T. Yoko, Thin Solid Films

306 (1997) 78-85.19. S.Y. Sohn, H.M. Kim, S.H. Park, and J.J. Kim, J. Korean

Phys. Soc. 44 (2004) 1215-1219.20. R. Cebulla, R. Wendt, and K. Ellmer, J. Appl. Phys. 83

(1998) 1087-1095.21. B.D. Cullity, Elementary of X-ray diffraction, 2nd Ed.

(Addison-Wesley, Reading, MA, 1978) p.102.22. K.S. Hwang, Y.S. Jeon, S.B. Kim, C.K. Kim, J.S. Oh, J.H.

An, and B.H. Kim, J. Korean Phys. Soc. 43[5] (2003) 754-

757.23. D. Shimono, S. Tanaka, T. Torikai, T. Watari, and M.

Murano, J. Ceram. Proc. Res. 2[4] (2001) 184-188.24. W.I. Park, and G.C. Yi, J. Electron. Mater. 30 (2001) L32-

L35.25. M.H. Choi, and T.Y. Ma, J. Mater. Sci. 41 (2006) 431-435. 26. D.C. Look, D.C. Reynolds, J.R. Sizelove, R.L. Jones, C.W.

Litton, G. Cantwell, and W.C. Harsch, Sold State Commun.105 (1998) 399-401.

27. C. Lin, C. Hsiao, S. Chen, and S. Cheng, J. Electrochem.Soc. 151[5] (2004) G285-G288.

28. K. Vanheusden, W.L. Warren, and C.H. Seager, J. Appl.Phys. 79[10] (1996) 7983-7990.

29. Y. Zhang, G. Du, D. Liu, X. Wang, Y. Ma, J. Wang, J. Yin,X. Yang, X. Hou, and S. Yang, J. Cryst. Growth 243(2002) 439-443.

30. B. Lin, Z. Fu, Y. Jia, and G. Liao, J. Electrochem. Soc.148[3] (2001) G110-G113.